Impatient snake's first time in the grass (5)
I believe the answer is:
hasty
'impatient' is the definition.
(I've seen this in another clue)
'snake's first time in the grass' is the wordplay.
'snake's first' becomes 's' (hst letter of 'snake').
'time' becomes 't' (abbreviation).
'in the' means one lot of letters goes inside another.
'grass' becomes 'hay' (hay is dried grass).
's'+'t'='st'
'st' going inside 'hay' is 'HASTY'.
(Other definitions for hasty that I've seen before include "Impetuous - quick-tempered" , "Too hurried, careless" , "Precipitate" , "Rushed; rash" , "Speedy" .)